Snails,Goniobasis livescens (Menke), were exposed to acutely sublethal concentrations of p-nitrophenol and the lampricide, 3-triflourmethyl-4 nitrophenol (TFM), and then to acutely lethal thermal shocks. The same spp were also exposed to acutely sublethal concentrations of Zn followed by exposure to acutely lethal concentrations of p-nitrophenol; and to acutely sublethal concentrations of TFM followed by exposure to acutely lethal concentrations of Zn. Brown trout, Salmo trutta, were exposed to acutely sublethal concentrations of TFM and then to an acutely lethal thermal shock. Results of these experiments indicate that prior exposure to acutely sublethal toxicant concentrations may reduce survival time for a subsequent exposure to acutely lethal concentrations of a second toxicant (e.g., snails exposed to Zn++ then p-nitrophenol) but neither prior or concomitant exposure to acutely sublethal toxicant concentrations insures that the median survival time for a lethal exposure will be significantly altered (e.g., snails exposed to 0.2 x 48 hour LC50 for TFM then Zn++). However, some acutely sublethal concentrations of a toxicant may significantly alter survival time of snails to a lethal concentration of a different toxicant (e.g., exposure to 0.4 x 48 hour LC50 TFM than a lethal dose of Zn++). The brown trout exposed to an acutely sublethal concentration of TFM and then an acutely lethal thermal shock did have significantly altered survival patterns.